Star Clusters
Open Clusters
Globular Clusters
An open star cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and are still loosely bound together by gravity. The Pleiades in the Northern Hemisphere is a beautiful naked-eye example of an open cluster.
Age
Open clusters are relatively young cosmic objects, typically ranging from a few million to a few hundred million years old. Because they are not tightly bound by gravity, their member stars tend to move away from each other. In hundreds of millions of years, as they orbit their galaxy's center, they may drift far enough apart that they will no longer be part of a cluster.
Number of Stars
These clusters can contain anywhere from a few dozen to a few thousand stars. A well-known example is the Pleiades (the Seven Sisters), which contains over 1,000 confirmed members and is about 100 million years old.
How to view
A open cluster is best viewed through a small to medium sized telescope. Even binoculars can be used to resolve the stars. A larger telescope may be used under a lower power. This way you can see the stars, but still be able to view the entire cluster in the eyepiece. As always, the darkest skies are best, but open clusters can be seen without a super dark sky.
A globular cluster is a stable, tightly packed, spherical collection of ancient stars that orbit a galaxy's core. Our Milky Way galaxy contains about 150 globular clusters. Omega Centauri contains upwards of 1 million stars. It is located in the Southern Hemisphere and is a great naked-eye object
Age
These are thought to be some of the oldest objects in the universe. A globular clusters typical age ranges from 10 to 13 billion years. This means they are some of the first structures to have formed in a galaxy and contain some of its most ancient stars. Their age is close to that of the universe itself.
Number of Stars
These clusters are incredibly dense and can contain anywhere from tens of thousands to millions of stars. This massive number of stars is all packed into a relatively small spherical region, held together by a strong mutual gravitational attraction. A famous example is Omega Centauri, which contains several million stars.
How to view
A globular cluster is best viewed through a moderately sized telescope. An 8" SCT, 4" Reflector or Refractor and a 4"-6" Dobsonian are good choices. Under dark skies, these telescopes should be able to show the core and resolve thousands of stars with the cluster.
M11, or the Wild Duck Cluster can be found near the constellation of Scutum. At about 220 million years old, it contains approximately 2900 stars. It was discovered by Gottfried Kirch in 1681.
The Double Cluster are two open clusters, NGC 869 and NGC 884. They are near the constellations of Perseus and Cassiopeia and visible to the naked-eye as two faint patches. They contain approximately 300 stars each.
M45, The Pleiades or The Seven Sisters, can be found near the constellation of Taurus. At about 100 million years old, it was first observed through a telescope by Galileo Galilei in 1610 and catalogued by Charles Messier in 1771
M54 is located in the constellation of Sagittarius. Discovered by Charles Messier in 1778. M54 is estimated to be about 13 billion years old. The Galactic Center is in the direction of Sagittarius. There are many other clusters within close proximity to M54
Also called The Great Hercules Cluster, M13 is located in the constellation of Hercules. It was discovered by Edmund Halley in 1714. M13 is said to be about 11 billion years old and contains over 1 million stars.
NGC 6934 can be found near the constellation of Delphinus. Founded in 1785 by William Hershel. Estimated to be about 10 billion years old and contains millions of stars.
